The present invention relates to an improved method for producing a silicon-on-insulator (SOI) wafer.
As known, according to widespread practice in the microelectronics industry, the substrate of integrated devices is obtained from wafers of monocrystalline silicon. In the last few years, as an alternative to wafers consisting of silicon alone, composite wafers, so-called xe2x80x9cSOIxe2x80x9d (Silicon-on-Insulator) wafers have been proposed, comprising two silicon layers, one of which is thinner than the other, separated by a silicon oxide layer.
A method for producing SOI wafers is the subject of European patent application no. 98830007.5 filed on Jan. 13, 1998 for the same applicant, and is described hereinafter with reference to FIGS. 1-9.
According to this method, and referring to the figures, on a surface 3 of a monocrystalline silicon region 2, initially a first silicon oxide layer is grown, having a thickness for example comprised between 200 and 600 xc3x85; a first silicon nitride layer having a thickness comprised between 900 and 1500 xc3x85 is then deposited. Using a resist mask, dry etching is carried out of the uncovered portions of the first oxide layer and the first nitride layer, and the resist mask is then removed, providing the intermediate structure of FIG. 1, wherein the wafer thus obtained is shown at 1, and the portions of the first oxide layer and the first nitride layer remained after dry etching, are indicated at 4 and 5 and define respective first protective regions 7, covering first portions 8xe2x80x2 of monocrystalline silicon region 2.
The first protective regions 7 form a hard mask, indicated at 9 and are used to etch the monocrystalline silicon region 2 at the second portions 8xe2x80x3 left uncovered by the mask 9, such as to form initial trenches 10 (FIG. 2), having a depth comprised between 0.5 and 5 xcexcm, depending on the desired characteristics of the buried oxide layer to be produced.
Subsequently, as shown in FIG. 3, the wafer 1 is subjected to oxidation, to form a second oxide layer 11, having a thickness, e.g. comprised between 200 and 600 xc3x85, and covering the lateral and base walls 10a, 10b of the initial trenches 10, and a second silicon nitride layer 12 having a thickness comprised between 900 and 1500 xc3x85 is then deposited.
Subsequently, layers 12 and 11 are anisotropically etched without a mask. Owing to the etching anisotropy, the horizontal portions of the second silicon nitride layer 12 and the oxide layer 11, on the base walls 10b of the initial trenches 10 and on the portions 4 and 5, are removed, thus providing the intermediate structure of FIG. 4. The regions 8xe2x80x2 are still covered on top by the mask 9, and laterally (on the vertical walls 10a) by oxide and nitride portions 11xe2x80x2 and 12xe2x80x2; on the other hand, the monocrystalline silicon region 2 is bare on the base wall IOb of the initial trenches 10.
The uncovered silicon, at the base wall 10b of the initial trenches 10 is etched, to deepen the initial trenches 10, obtaining final trenches 16 having a required depth. In particular, the depth of the final trenches 16 (like that of the initial trenches 10) determines the dimensions of the desired covered oxide layer, and thus the electrical characteristics of the SOI wafer, as explained hereinafter, and is thus selected according to the specifications provided for the final SOI wafer.
The monocrystalline silicon region now comprises a base portion shown at 2xe2x80x2, and a plurality of xe2x80x9ccolumnsxe2x80x9d 18, extending vertically from the base portion 2xe2x80x2. Thus the intermediate structure of FIG. 5 is obtained, wherein the nitride portions 5 and 12xe2x80x2 are no longer separate from each other, and are indicated at 19, and the oxide portions 4 and 11xe2x80x2 are also no longer separate from each other, and are indicated at 20 and, with the portions 19, form second protective regions 30.
A thermal oxidation step is then carried out, thereby the exposed silicon regions of the xe2x80x9ccolumnsxe2x80x9d 18 are transformed into silicon oxide. In practice, the oxide regions gradually grow from the silicon regions, from the lateral walls of the final trenches 16 towards the interior of the columns, and partially also towards and inside the base portion 2xe2x80x2. Since during the oxidation the volume increases, the oxide regions that are being formed gradually, occupy the space of the final trenches 16, until they close the latter completely, and are joined to one another. The oxidation step ends automatically once the columns 18 have been completely oxidized (apart from the upper area or tip, indicated at 21, which is protected by the second protective regions 30), thus forming a continuous buried oxide region 22, shown in FIG. 6, wherein continuous vertical lines show meeting surfaces of the oxide regions being formed from walls of two final adjacent trenches 16, showing the oxide expansion.
Subsequently, by selective etching, the second protective regions 30 are removed and uncover the xe2x80x9ctipsxe2x80x9d 21, which are designed to form the nuclei for a subsequent epitaxial growth.
The structure of FIG. 7 is obtained, showing the three-dimensional structure of the wafer I in this step. Subsequently, epitaxial growth is carried out, the parameters whereof being selected to avoid silicon nucleation in the areas above the buried oxide region 22, and to have a high ratio of lateral to vertical growth, to obtain first horizontal growth of silicon around the tips 21 (thus covering the upper surface of the buried oxide region 22), and then vertical growth of an epitaxial layer 23. After an optional step of chemical/mechanical polishing to level the upper surface of the wafer 1, the final structure of the wafer 1, shown in FIG. 8, is then obtained.
Thereby it is possible to produce an SOI wafer using only process steps common in microelectronics, with far lower costs than those of the processes used now for production of SOI substrates.
However, the above-described production method has the disadvantage that during the step of non-masked anisotropic etching, there are uncovered oxide portions, which, during the subsequent epitaxial step, produce extensive areas rich in crystallographic defects.
In particular, as illustrated in greater detail in FIG. 9, relative to a single initial trench, the second oxide layer grows, inter alia, towards the interior of the initial trench 10, forming steps 39. Consequently, when subsequently depositing the second silicon nitride layer 12, the latter follows the inner profile of the lateral walls of the initial trenches 10, and thus in turn forms a pair of steps 40 in each initial trench 10.
The presence of these steps 39, 40 means that in the subsequent step of non-masked anisotropic etching of the layers 12 and 11, not only the horizontal portions of the second silicon nitride 12 and oxide 11 layers are removed on the base of the initial trenches 10, and above the portions 4 and 5, but also the portions of the second silicon nitride layer 12 which form the steps 40, thus leaving uncovered the portions of the second oxide layer 11xe2x80x2 defining the steps 39, as illustrated in FIG. 10.
Consequently, in the subsequent oxidation step for growing the buried oxide region 22, silicon oxide is grown at the steps 39, forming oxide areas 41 (FIG. 11). These oxide areas 41 cause the formation, in the subsequent epitaxial growth, of extensive areas rich in crystallographic defects in the epitaxial layer 23, which areas are delimited by broken lines in FIG. 12.
The present invention improves the above-described method so as to eliminate the crystallographic defects present in the epitaxial layer and to obtain improvement of the electrical characteristics of the SOI wafer.
According to the disclosed embodiments, a process for producing a wafer includes forming a protective layer over selected portions of a substrate, the protective layer comprising a non-insulating layer and an insulating layer formed between the noninsulating layer and the substrate; forming a first trench in non-selected portions of the substrate, the first trench having lateral walls and a base wall; and removing surface area from the lateral walls of the first trench and from end portions of the insulating layer to form a second trench from the first trench.